Transport refrigeration units are used to maintain a desired temperature in a conditioned load space inside an enclosure used for carrying cargo, such as in a trailer, truck or other transport container. A transport refrigeration unit can be installed on the exterior of the enclosure, outside of the conditioned load space. A typical cargo container is a truck, and a typical mounting site for the transport temperature control unit is above the truck cabat the front wall of the enclosure.
Transport refrigeration units generally include an evaporator assembly that transfers heat from the conditioned load space into a refrigerant, and a condenser assembly that transfers heat from the refrigerant to the outside environment. The evaporator assembly typically includes an evaporator coil and an air-moving apparatus (e.g., a fan). The air-moving apparatus draws relatively warm air from the conditioned load space, passes the air over the evaporator coils, which take heat from the air and return the cooler air to the conditioned load space. The condenser assembly typically includes condenser coils and an air-moving apparatus (e.g., a fan), which draws air from the outside environment over the condenser coils and returns the heated air to the outside environment.
Transport refrigeration units also generally include a refrigerant compressor to pressurize the refrigerant and an expansion valve to depressurize the refrigerant. The evaporator assembly, condenser assembly, compressor and other components in the temperature control unit require a power supply. Conventional transport refrigeration units employ an engine, such as an internal combustion diesel engine, to supply the needed power (for the compressor, fans, valves, etc.). The engine can be separate from the vehicle engine or the vehicle engine itself can be used. If the vehicle engine is used, electrical connections need to be made between the refrigeration unit and the engine""s electrical source, usually an alternator. In addition, some units utilize a compressor driven directly by the vehicle engine, requiring pipe connections from the compressor to the refrigeration unit. The potential for leakage or for electrical problems is increased with the increased distance between the refrigeration unit and the engine. Other units utilize a separate engine mounted near the refrigeration unit. This eliminates the leakage problems but introduces new problems. The engine will require additional maintenance and fuel to operate, increasing the costs of operating the unit. Typical vehicle engine-driven air conditioning systems have included inverter circuit components. However, these systems are power supply driven, where the output frequencies of the inverters are adjusted in response to the power supplied to the unit.
It is generally desirable to make the transport refrigeration unit as compact and as efficient as possible. Both objectives can be advanced by making the powered components of the unit electrically-powered and independently controlled. By making compressor, condenser fans and evaporator fans electrically-powered and independently-controlled, there is no need for a mover in the transport refrigeration unit. Generally speaking, a mover is a device that uses mechanical or chemical energy to drive another component. In the case of a transport refrigeration unit, a mover could include a diesel engine. Frequently a mover drives another device mechanically, by means such as belts and pulleys. A mover and the mover""s associated apparatus consume considerable space. If the mover is a diesel engine that drives an electric motor for example, the engine and the motor both take up space, as do the belts and pulleys and other mechanical driving systems.
Components in a transport refrigeration unit that are electrically-powered and independently-controlled can utilize the already existing, and required, vehicle engine as a mover. Using the already existing electrical system allows for the use of efficient self-contained fans and compressors. A condenser fan, for example, can include its own inductive motor, and need not be mechanically driven by a mover. Compressors such as hermetic scroll compressors likewise can include their own electric motors.
However, using the electricity generated by the vehicle engine may be inefficient due to the wide variations in frequency and voltage that occur during normal operation. For example, as the vehicle engine speed increases or decreases, the frequency of the electric power from the corresponding alternator fluctuates. Therefore it is desirable to control the power supply to the differing components to optimize the efficiency of the cooling unit given the limited amount of power that may be available.
Therefore the present invention provides a method of powering a refrigeration system is provided. The method includes providing a mover, and providing an alternator, the alternator being coupled to the mover, and generating a power signal. The method further includes monitoring at a control a plurality of system parameters, and sending a control signal based on the system parameters from the control. Furthermore, the method includes receiving the power signal and the control signal at an inverter-based device, the inverter-based device having a plurality of inverters, and converting the power signal into a controlled power signal based on the control signal. The method also includes driving a plurality of components of the refrigeration unit with the controlled power signal, the components also being controlled by the control.
In another embodiment, a method of power distribution in a temperature controlled transport unit is provided. The method includes providing a primary power signal and converting the primary power signal into a secondary controlled power signal with a plurality of inverters. The inverters are coupled to a control that sends control signals to the inverters. The method further includes driving a plurality of components in the temperature controlled system with the controlled power signal. The components are also controlled by the control.
In still another embodiment according to the present invention, a power distribution system in a temperature controlled transport system is provided. The system includes a mover operatively coupled to an alternator that generates a power signal. The mover is also operatively coupled to a control that monitors a plurality of system parameters and sends a control signal based on the system parameters. The system also includes an inverter-based device operatively coupled to the control, the inverter based device having a plurality of inverters, receiving the power signal and the control signal, and converting the power signal into a controlled power signal based on the control signal. Furthermore, the system includes a plurality of components, the components being driven with the controlled power signal and being controlled by the control.
In the preferred embodiments, the present invention utilizes an alternator coupled to the vehicle engine to provide an alternating current (xe2x80x9cACxe2x80x9d) power signal. Rectification of the AC power signal creates a direct current (xe2x80x9cDCxe2x80x9d) power signal that is passed through a DC bus voltage controller and then supplied to a pair of inverters. The DC bus voltage controller controls the variable voltage generated by the alternator. Each inverter converts the DC power signal into a controlled AC power signal for driving the components of the refrigeration unit. One AC power signal can be used to drive the compressor, which requires the largest amount of power. The second AC power signal is then used to power the evaporator fan and condenser fan. This arrangement allows for the motors to be run at different speeds and power levels depending on the amount of cooling required and the amount of power that is available from the engine.
The alternator is liquid cooled, with the coolant heat rejected using a heat exchanger incorporated in the refrigeration system condenser coil or, alternatively, in the refrigeration system evaporator coil. The increased cooling efficiency resulting from the use of the refrigeration system heat exchanger or evaporator coil, as opposed to using the vehicle radiator heat exchanger, enables the use of a smaller alternator.
A microprocessor-based control receives and processes a number of input variables and runs a control algorithm to efficiently manage the power supply and electrical load of the system. The control continuously monitors refrigeration system parameters including alternator speed, refrigeration system pressure, watt power transducer values, current power transducer values, refrigeration system suction pressure value, fixed suction pressure value, and condenser and evaporator discharge temperature to determine the current electrical load of the system. The control algorithm continuously establishes: (1) the position of a suction line proportional refrigeration valve; and (2) the inverters"" output frequency/voltage. Together, these controlled parameters establish the alternator input power. A predetermined, prime mover speed-dependent utilizable power map is incorporated in the control algorithm. The alternator""s input power consumption is made equivalent to the prime mover speed-dependent utilizable power map.
The control algorithm also controls the refrigeration unit high side refrigerant pressure at extreme conditions. A high pressure control set point is input into a control algorithm. Rising refrigerant pressure results in changing the compressor speed with Proportional Integral Derivative (xe2x80x9cPIDxe2x80x9d) control to limit the pressure to the set point value. Further required reduction in refrigerant pressure uses a suction line PID controlled refrigerant valve. Other unit performance parameters are continuously monitored to allow for further changes in compressor speed or refrigeration unit suction flow. The control algorithm prevents exceeding the refrigeration pressure set point limit and avoids shut down of the refrigeration unit.
The control algorithm incorporates a soft start power management function. The rate of application of load power to a vehicle engine influences drivability of the vehicle. Using an established load application rate, measured in watts per second, minimizes the influence on vehicle performance. Determination of an acceptable application rate for each vehicle or a rate for all vehicles, optimizes the performance of a vehicle powered refrigeration product, deriving power from a vehicle.
Further objects and advantages of the present inventive transport refrigeration system, together with the organization and manner of operation thereof, will become apparent from the following detailed description of the invention when taken in conjunction with the accompanying drawings, wherein like elements have like numerals throughout the drawings.
Other features and advantages of the invention will become apparent by consideration of the detailed description and accompanying drawings.